Migration of conventional embolization coils occurs 4-14% of transcatheter embolizations [2,3]. Non-target embolization is an outcome of coils migration, the impact of which depends on the final location of the coils. In the venous system, the consequences can be catastrophic with literature indicating that coils can migrate into the renal vein, right atrium of the heart, lung (pulmonary artery). Percutaneous retrieval of the coils is technically very challenging and frequently cannot be attempted as the coils are often entrapped within the organs and tissue.
Coil migration occurs for various reasons:                Technical error: release of a coil or coil pack too distal or proximal to an adjoining larger vessel or plexus [6,7]        High blood flow areas can cause the coil to migrate.        Coil: vessel mismatch. The coils are undersized, hence will not injure the vessel wall, will not induce thrombosis, and are likely to migrate. Or the coils are oversized and will act like a guide-wire and pass further distally into the vessel [8,9].        Vessel dilation: coil migration can occur due to a disparity in the size of coils and dilated vessels, which can change in their diameters depending on vessel hemodynamics [5].        Coils impart a very low radial (anchor) force on the lumen, once a clot forms within the coil, blood flow can force it to migrate.        
The profile of the embolization device and delivery system is a critical success factor in successfully accessing target embolization locations e.g. the iliac arteries are frequently tortuous in the presence of abdominal aortic aneurysms [8]. To combat this issue today, microcatheters are often employed in difficult or tortuous anatomy where use of standard catheters may induce spasm and lead to a failed embolization procedure [8]. Additionally different stages in a procedure may require catheters with different mechanical properties e.g. accessing a visceral vessel, particularly in the presence of diseased or tortuous arteries, may require a catheter with a high degree of stiffness and torque control. In general, the lower the profile of the device and delivery system, the greater the accessibility of the device into tortuous and higher order vessels. A lower profile device reduces the diameter of catheter required for delivery and lowers the risks of access site infections, hematomas and lumen spasm.
Dependent on the clinical application of the device, variable anchor forces may be required to prevent migration of the prosthesis e.g. arterial and venous applications have variable blood flow rates and forces. This in turn, will lead to a compromise in terms of profile since larger fibres, which better anchor the bristle device in the lumen, will require a larger catheter for delivery.
The technique generally used to embolise vessels today is to insert a metallic scaffold (coil, plug) into the target vessel, to cause a thrombus that adheres to the scaffold, relying on the thrombus to induce blood cessation and eventually occlude the vessel. In general, available embolization technology does not interfere with or interact with blood flow densely enough across the vessel cross section to induce rapid, permanent vessel occlusion.
Using technology available today, the physician will often have to prolong a specific duration of time for the technology to induce occlusion. In one approach the physician inserts coils and then waits 20 minutes for the coils to expand and cause vessel occlusion [1].
The restoration of the lumen of a blood vessel following thrombotic occlusion by restoration of the channel or by the formation of new channels, is termed recanalisation. Recanalisation can occur due to, coil migration, fragmentation of the embolisation material, and formation of a new vessel lumen that circumvents the occlusion [9]. Recanalization rates vary by procedure and embolic agent, ranging from 10% to portal vein embolization to 15% for pulmonary arteriovenous malformations to 30% for splenic artery embolization [12,14,15]